Phenylalanine quantitative analysis by gas chromatography-mass spectrometry (Phenylalanine, Quantitative by Gas Chromatography Mass Spectrometry - GC-MS) is a precise test used to measure phenylalanine levels in plasma. This test is a vital tool in diagnosing and monitoring phenylketonuria (PKU) and other metabolic disorders related to amino acids. The analysis is highly accurate due to the use of advanced techniques such as gas chromatography and mass spectrometry, making it a gold standard in the field. A blood sample is drawn and analyzed in a specialized laboratory, and the results are essential for assessing the patient’s health and determining the need for dietary or therapeutic interventions. This test is recommended in cases with a family history of metabolic disorders or when specific symptoms appear.